TOKYO—The Japanese government initially underestimated radiation releases from the Fukushima Daiichi nuclear plant, in part because of untimely rain, and so exposed people unnecessarily, a report released this week by a government research institute says.

Adding to earlier evidence of initial government missteps, the report by the Japan Atomic Energy Agency saysan unlucky combination of heavy rains and shifting winds meant that much of the airborne radioactive debris washed down over a broad area around the crippled plant. Before the changing weather, the radiation had been expected to drift over the Pacific Ocean, which would have posed less of a risk to public health, at least in the short term.

“Local residents would have stayed indoors and avoided radiation if they had been told about the dangers of the rainfall,” said Tetsuo Sawada, assistant professor of reactor engineering at the Tokyo Institute of Technology.

The Japanese government’s initial evacuation zone—after the March 11 earthquake and tsunami knocked out the plant’s cooling systems and caused core meltdowns—was within 20 kilometers (about 12 miles) of the stricken plant. But as the study highlights, radiation spread far beyond the 20-kilometer radius, with rainstorms contributing to the ground contamination.

According to the agency, the rain came on the worst possible day for plant operators—March 15, the day an explosion struck the plant’s No. 2 reactor, punching a large hole in the suppression chamber that is part of the primary containment vessel, the main shield for radiation releases. The gash allowed toxic air to leak into the atmosphere without check.

According to the government’s Nuclear and Industrial Safety Agency, radiation releases peaked around that day, dropping as workers managed to cool down the badly damaged No. 2 unit, as well as the other three seriously damaged reactors.

“If there was no rain on March 15, the ground contamination would have been far less severe than it is,” said Haruyasu Nagai, an author of the report.

Two earlier explosions, just after the disaster, at reactors Nos. 1 and 3, didn’t release nearly as much radiation because they occurred outside the primary containment vessels. The explosion at No. 2, by contrast, was caused by a buildup of pressure inside the containment vessel, as the overheating reactor kept producing steam.

The rain started falling in areas around the plant in the afternoon of March 15. At the same time, the wind, which had been heading east—as is normal for the season—shifted and started heading northwest, carrying the toxic air deep into the country.

By the time the rain stopped, a large swath of land to the northwest of the plant, well beyond the 20-kilometer radius, was contaminated far more than allowed for human habitation. In late April, the government belatedly decided to evacuate residents in these areas.

“Much of the radioactive substance would have been carried into the ocean on an easterly wind eventually,” says the report’s author, who estimates about half of the radiation released in March ended up falling into the ocean.

Asked about the latest report, a spokesman for the nuclear-safety agency said the results appeared to be valid. “The radiation is likely to have spread as the JAEA analysis suggests,” Yoshinori Moriyama said.

… these are not “dosimeters” but “glass badges” that passively collect radiation information. It won’t help these children or their parents to avoid high-radiation areas and spots, it won’t tell them how much radiation they will have been exposed unless they are sent in to a company to interpret the data.

Radiation exposure is increased by a factor of a trillion. Inhaling even the tiniest particle, that’s the danger.

Yo: So making comparisons with X-rays and CT scans has no meaning. Because you can breathe in radioactive material.

Hirose: That’s right. When it enters your body, there’s no telling where it will go. The biggest danger is women, especially pregnant women, and little children. Now they’re talking about iodine and cesium, but that’s only part of it, they’re not using the proper detection instruments. What they call monitoring means only measuring the amount of radiation in the air. Their instruments don’t eat. What they measure has no connection with the amount of radioactive material.

Dr. Helen Caldicott (Co-founder of Physicians for Social Responsibility):

You’ve bought the propaganda from the nuclear industry. They say it’s low-level radiation. That’s absolute rubbish. If you inhale a millionth of a gram of plutonium, the surrounding cells receive a very, very high dose. Most die within that area, because it’s an alpha emitter. The cells on the periphery remain viable. They mutate, and the regulatory genes are damaged. Years later, that person develops cancer. Now, that’s true for radioactive iodine, that goes to the thyroid; cesium-137, that goes to the brain and muscles; strontium-90 goes to bone, causing bone cancer and leukemia. It’s imperative … that you understand internal emitters and radiation, and it’s not low level to the cells that are exposed. Radiobiology is imperative to understand these days.”The Japanese people are used as guinea pigs: